1. Field of the Invention
The present invention relates generally to communications, and in particular but not exclusively, relates to constellation mapping and bit loading in multi-carrier transceivers.
2. Background Information
High-speed data communication paths are typically required in order to make high data rate services, such as video and Internet access, available to end users. Fiber optic communication paths are well suited for these high data rate services, but are not readily available in existing communication systems and are expensive to install. Therefore, asymmetric digital subscriber line (ADSL) technology has been developed to allow use of current telephone wiring connections (e.g., twisted-pair copper lines) for high data rate services.
ADSL systems typically use discrete multi-tone (DMT) techniques (e.g., a multi-carrier technique) that divide the available bandwidth of twisted-pair copper lines into multiple channels or xe2x80x9cbins.xe2x80x9d With DMT, a plurality of frames of a data stream is broken down into data blocks. Each data block is allocated to multiple carrier channels. A carrier channel, in turn, can be represented as a vector whose magnitude and phase is based on the data that the carrier channel is carrying and on the amount of bits that the carrier channel can support (sometimes referred to as xe2x80x9cbit loadingxe2x80x9d or xe2x80x9ctone orderingxe2x80x9d). The bit loading of the carrier channel is indicative of the number of constellation points (e.g., the number of magnitude and phase combinations for the vector). Thus, if the bit loading of a particular carrier channel is 2, then the number of constellation points is 4, with a constellation point in each quadrant representing the binary number 00, 01, 10, or 11 for example. This process of associating binary numbers to constellation points is sometimes referred to as xe2x80x9cconstellation encodingxe2x80x9d or xe2x80x9cconstellation mapping.xe2x80x9d
Each of the carrier channel vectors may be used to produce a quadrature amplitude modulated (QAM) signal at a given frequency. The QAM signals are then summed to produce a time domain DMT xe2x80x9csymbolxe2x80x9d that is subsequently transmitted over the twisted-pair copper line. That is, each of the carriers that make up the DMT symbol contains a QAM signal. A DMT symbol is generated for each frame of the original data stream.
There are drawbacks, however, with the techniques described above. Current ADSL systems typically mix/multiplex voice, data, video, etc. in the same processing path, using the same constellation encoding and bit loading for these different types of information. Thus voice and data are handled/processed equally without distinguishing one from another. For example, aggressively bit loading so as to provide a constellation having many points produces a high bit rate but results in a high error rate. Conversely, conservatively bit loading so as to provide a constellation having fewer points produces a lower bit rate but results in a low error rate. While voice transmission is more tolerant of errors, data transmission is not. By treating voice and data equally in terms of bit loading, existing ADSL systems do not take such factors into account, thereby resulting in non-optimal performance of the system.